Many devices need "good" p-n junctions made from semiconductors having a wide band-gap. A "good" p-n junction provides acceptable carrier injection accross the junction and has low series resistance in the bulk portions. "Good" p-n junctions also require adequate conductivity in both n- and p-type materials.
For example, to produce light efficiently with a semiconductor, the energy of the light emitted is less than or at most, equal to the energy of the band-gap. Because the band-gap of Si is small about (1.1 eV) it is impossible to emit blue or green light efficiently from this semiconductor The energy of the band-gap is too small.
TABLE 1 ______________________________________ Energy of Band Gap of Several Wide Band-Gap II-VI Semiconductors ______________________________________ CdS 2.4 eV ZnO 3.0 eV CdSe 1.7 eV ZnS 3.7 eV CdTe 1.4 eV ZnSe 2.7 eV ZnTe 2.3 eV ______________________________________
Considerable effort has been expended in attempting to develop semiconductors having both high bipolar conductivity and a wide band-gap. The energy's of several wide band-gap II-VI semiconductors is set out in Table 1. Despite this effort, suitable semiconductors of this type have not been developed. Although the "n-type" side or "p-type" side of these semiconductors often has low resistivity, at least one side has always had unacceptably high resistivity. For example, a II-VI semiconductor such as n-type ZnSe has both a wide band-gap and low resistivity. However, p-type ZnSe exhibits unacceptably high resistivity. Further, n-type ZnTe which also has a wide band-gap likewise has high resistivity.
The cause of high resistivity in either the p-type or the n-type of semiconductors having wide band-gaps is due to compensation. Compensation refers to the phenomenon in which electron donors are unintentionally present in what was intended to be p-type semiconductor or acceptors are unintentionally present in what was intended to be an n-type semiconductor. The heretofore unavoidable presence of acceptors or donors on the "wrong" side of a p-n junction in wide band-gap semiconductors unacceptably limits the utility of p-n junctions formed from materials having a wide band-gap.
Considerable effort has been expended to develop wide band-gap semiconductors having low bipolar resistivity. A general goal has been to develop p-n junctions having bipolar resistivity less than about 200 to 100 ohm.multidot.cm in either direction. Efforts to achieve these goals have included the use of molecular beam epitaxy (MBE), but this involves extremely expensive equipment and processes and any reported successes have not been suitably reproduced. Other efforts have included metal organic chemical vapor deposition (MOCVD) but again this procedure is extremely expensive and has not produced suitable results reliably.